Gas detector and analyzer

ABSTRACT

A portable, self-contained gas detector and analyzer is disclosed which includes a carrier gas supply, and electrical power supply, a sampling loop and a chromatographic column with an electron-capture detector. A preferred embodiment is adapted to respond to a pre-selected tracer gas. If the tracer is included in a closed system whose integrity is suspect, the detector can first be operated to detect the presence of the tracer and then can signal the rate at which the tracer is being provided to a predetermined, limited volume.

United States ?atent Josias et a]. 1 1 Jan. 30, 1973 [541 GAS DETECTORAND ANALYZER 3,450,877 6 1969 Zimmer ..250 43.5 MR

Inventors: Conrad S- JOSiaS; D. B0wman, both of Los Angeles, Cal1f.;James LovelockWiltsElmland Ion1zat1on Methods for the Analys1s of Gasesand "a H .7 V 7 .ma a oro, Vapors, by Lovelock, J. E., from AnalyticalChemis- [7 Assignee= Analog Technology Corporation, try, v61. 33, NO. 2,Feb. 1961, pgs. 162 to 177.

Pasadena, Calif., by said Lovelock Primary Examiner-Archie R. Borchelt[22] May 1969 AttorneyGolove 81. Kleinberg [21] Appl. No.: 835,290

[57] ABSTRACT [52] US. Cl ..250/43.5 MR, 73/23.l, 250/836 FT A portable,self-contained gas detector and analyzer is [51] Int. Cl. ..G01n 23/12disclosed which includes a carrier gas supply, and [58] Field of Search..250/43.5 MR, 44, 83.6 FT; electrical power supply, a sampling loop anda chro- 73/23, 23.1 matographic column with an electron-capturedetector. A preferred embodiment is adapted to respond to [56]References Cit d a pre-selected tracer gas. If the tracer is included ina closed system whose integrity is suspect, the detector UNITED STATESPATENTS can first be operated to detect the presence of the 3 298 788 11967 Dewar et al. ..73 23 X and can signal the rate at which the3:489:903 1/1970 Robinson ..250/83.6 FT is being Provided to apredetermined, limited 3,361,908 1/1968 Petitjean et al ..250/43.5 MR

10 Claims, 11 Drawing Figures PAIENTED JAN 30 1973' SHEET 10F 7 James E.Loveldck an... l %m. S J R T S dDEY. VB mdN fl Ob CL GOLOVE 8 KLEINBERG,

ATTORNEYS.

PATENTEDJAN 30 1975 Range Multiplier set at I00 Range Multiplier set atl0 Range Multiplier set at l Milliammflor Read in 3.714.421 SHEET 50F 7Fig. 80.

O-Ol lllllll l||||l|| 2 3 456789lo 2 3 4 56789 2 v W xlo x|o' x|0' Leakrate (atm. cc/sec) ice/sec. flow rate GAS DETECTOR AND ANALYZER Thepresent invention relates to the detection and measurement of lowconcentrations of gases and vapors and, more particularly, to portable,self-contained apparatus that is responsive to the presence ofparticular gaseous compounds which are used as tracers.

1n the prior art, devices have been taught for the qualitative andquantitative analysis of certain classes of organic compounds such ashalogenated hydrocarbons. One example of apparatus is shown in thepatent to Strange, US. Pat. No. 3,009,097. Another was disclosed in US.Pat. No. 3,247,375, which issued to James E. Lovelock on Apr. 19, 1966.Lovelock taught improved electron capture detectors which could signalthe presence of very low concentrations of several different compoundsthrough appropriate calibrations.

One specific embodiment, illustrated in FIG. 4 of the patent, employedan ionization chamber. An electrometer circuit measured variations incurrent flowing through the ionization chamber resulting from thecapture of free electrons by the compounds to be detected.

In an alternative embodiment, a pulse generator which provided positivepulses of amplitudes between 50 and 150 volts and widths between 0.5 andmicroseconds was connected across the electrodes of the ionizationchamber. The interval between pulses was approximately twice as long asthe width of the pulse. The signal output of the electrometer wasrecorded.

In a paper titled Electron Absorption Detectors And The Technique ForTheir Use In Quantitative and Qualitative Analysis By GasChromatography," published in Analytic Chemistry, Vol. 35, No. 4, April1963, p. 474481, prepared for the Baylor University College of Medicineat Houston, Texas, James E. Lovelock suggested an improvement over theapparatus disclosed in his patent. A pulse generator applied an input toone terminal of the detector and the other terminal of the detector wasconnected through an appropriate RC network to an electrometer. The

pulse generator provided pulses of approximately 50 volts amplitude andwidths between 0.5 and 1.0 microseconds. However, the period betweensuccessive pulses ranged between 5 and 200 microseconds. In thisarrangement, the sampling pulse collected substantially all of the freeelectrons at the anode, without any significant movement of eitherpositive or negative ions. The pulses were then integrated in the RCnetwork to provide a steady, DC current to the electrometer circuitwhich, in turn, provided an output to suitable recording devices.

Gas detectors and analyzers have, in the past, been large, bulky,relatively permanent installations requiring a substantial electricalpower supply and a substantial source of carrier gas for use withchromatographic systems. Most systems have been designed to work in alaboratory or in a fixed installation where these requirements did notconstitute a particular burden.

However, it has been deemed desirable to provide a lightweight,portable, completely self-contained device which can alternativelydetect the presence of a preselected tracer gas in an environment bymeasurementof concentration sensitivity in that environment,

or, to determine the rate at which a preselected tracer gas is enteringa limited volume, for the purpose of testing the integrity of closedsystems.

According to the present invention, there is provided in a smallsuitcase" size package, a completely portable, self-contained detectionapparatus that is sensitive to the presence of sulphur hexafluoride (SFwhich is a non-radioactive, non-toxic, inert tracer gas. Alternatively,the apparatus could be sensitive to freon, or other gases which have ahigh affinity for free electrons, for example, the halogen compounds.

The invention includes an improved gas handling sub-system, an improvedelectrical sub-system for detecting the preselected gas, and an improvedtest probe that permits the detection of the presence of the tracer inan environment as well as a leak detector for use with systems that aresupposed to be closed, that can signal the existence of a leak andwithin certain limits, the rate of leakage.

The gas handling sub-system includes a self-contained supply of acarrier gas, preferably argon with 10 percent methane (CH,,) and a gaspath which passes through a filter for contaminants, through asamplevalve system into a chromatographic column and then into anelectron-capture detector. For leak detection purposes, provision ismade to bypass some of the carrier gas to the probe.

The sampling path includes the sampling probe, a fixed-volume samplingloop which communicates with the sampling valve and then to a pump whichexhausts to the atmosphere. A flowmeter can be provided in the samplingpath and, to enable calibration and certain leak-rate measurements, abypass with an adjustable restrictor is provided in parallel with thepump.

The electrical sub-system includes an electron-capture detector which isconnected to a pulse generator that provides brief" sampling pulses(approximately 1 microsecond) at a relatively slow rate (approximately10,000 pulses per second). The output of the detector is adjustablebiased to null the output of the detector, when'carrier gas only isflowing.

The output of the detector is applied to an electrometer-amplifier whichis also provided with an adjustable bias, as well. A feedback loop isprovided with alternatively selectable paths. In addition to a normalfeedback path, a short circuit path is provided for nulling theamplifier, and two integrating paths are provided so that the leak ratecan be expressed by a relatively steady state signal at more than onelevel of sensitivity.

The output of the electrometer amplifier is adequate to drive a metermovement, a recorder instrument, or the output can be applied to avoltage controlled oscillator for generating an audio signal, thefrequency of which provides the significant information.

In the most sensitive mode of operation, the carrier gas supply isconnected to the carrier gas path. After a reasonable warm-up time, theshort-circuit in the feedback loop is connected in order to zero theamplifier. The built-in pump is energized to draw air samples throughthe probe and the sampling loop.

After setting the instrument for the desired sensitivity, a samplingbutton is depressed. The sample loop is then included in the carrier gasstream. The probe and pump are directly connected through a bypass path.At

the same time, the amplifier may be disable for a predetermined intervalof time, during which the oxygen component of the air sample passesthrough the detector. The amplifier is enabled during the time that theselected tracer gas would be eluted from the column and, if the traceris-present, a meter deflection will occur sometime between 60 and 120seconds after the sample has been included in the carrier gas path.Should an audio output signal be desired, a basic audio signal can bechanged in pitch in proportion to the meter deflection.

If the rate of a leak is to be determined as in the following examplewhich is one of several alternative methods, the gas flow paths areslightly modifier. A supply of carrier gas is furnished to the probe.The probe has a resilient collar which can be held securely against thearea to be tested. The resilient collar permits a substantiallyair-tight seal to be achieved. Appropriate sealing materials such asilicon grease can be applied to the resilient collar to improve theintegrity of the seal. An alternative method of testing involves placingthe vessel under test in a bag or other type of container and sealingthe container around the probe and vessel.

The volume of interest is continually being swept by a known volume ofcarrier gas which is flushed into the probe intake and through thesample loop. The sample button is energized and the sample loop isincluded in the primary carrier gas path. At the appropriate time, themagnitude of either deflection can be related to the rate of leakage,using appropriate calibration charts which have been prepared.

Should it be desired to measure the area of a peak (the area being themost accurate quantitative measurement), one of the alternativeintegrator" modes may be used. An integrating capacitor is included inthe selected amplifier feedback loop. The oxygen peak is firstautomatically blanked and then the tracer peak current is integratedacross the feedback capacitor. The output voltage is sustained withinconstraints of amplifier and baseline current drifts. By use of anoutput recorder, permanent records of integrated peaks may be made, andassociated drifts (which would ordinarily make the reading of low-levelmetered integrations difficult) can be subtracted. Appropriate chartspermit conversion of both meter and recorder readings to leak rates.

In its continuous monitoring mode, air is drawn in through the probe bya pump and a fraction of the sample is mixed in with the carrier stream.The mixture of carrier gas and attenuated sample is routed directly tothe detector, whose baseline current is reduced by the oxygen componentof air. The detection of a tracer component in theair sample furtherreduces the detector current in such a way that the measured outputbecomes a slowly varying dc value modulated by the local concentrationof tracer gas. This configuration provides comparatively fasteranalysis, but with less sensitivity.

The novel features which are believed to be characteristic of theinvention, both as to organization and method of operation, togetherwith further objects and advantages thereof will be better understoodfrom the following description considered in connection with theaccompanying drawings in which several preferred embodiment of theinvention are illustrated by way of example. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended as a definitionof the limits of the invention.

FIG. 1 is a front view of an instrument according to the presentinvention with the cover removed therefrom;

FIG. 2 is a view of the controls on the back of the instrument of FIG.1;

FIG. 3 is a diagram of the gas handling system of the present invention;

FIG. 4 is a block diagram of the electrical system of the presentinvention;

FIG. 5 is a side sectional view of an improved probe according to thepresent invention;

FIG. 6, including FIGS. 6a and 6b, is a partly idealized view of asampling valve for use with the present invention in both of itsalternative configurations;

FIG. 7 is a block diagram of the power supply of the instrument to thepresent invention;

FIG. 8 including FIGS. 8a and 8b are typical calibration charts forrecorder signals; and

FIG. 9 is a typical chromatogram derived from the system of the presentinvention.

Turning first to FIG. I, there is shown a portable, self-containedinstrument 10 according to the present invention. A dust cover (notshown) is provided to protect a front panel 12 from the normal hazardsof handling and transportation. A carrying handle 14 makes the deviceeasily transportable.

An improved probe assembly 16 is coupled by flexible tubing 18 to thegas-handling sub-system. A pliable, resilient collar 20 is mounted onthe probe 16 to enable reasonable gas-tight seals over limited areas ofinvestigation. v

On the front panel 12, there are located the various operating controlsof both the gas and electrical subsystems as well as appropriatedisplays by which various quantities can be measured. The variouselements of the front panel 12 will be identified here, but theirpurpose and function will be explained in greater detail in connectionwith the gas and electrical sub-systems which are described below. Acarrier gas valve 22 controls the flow of bottled, carrier gas and apressure gauge 24 is provided to indicate the current state of thecarrier-gas supply.

A probe flow control 26 is provided to divert a limited amount of thecarrier gas through the probe assembly 16 for leak-rate measurements.The main flow of carrier gas goes through a sampling valve control 28which, when energized selectively includes in the carrier gas path, apredetermined volume sample loop' through which has been circulating thegas to be tested as explained below. The carrier gas continues through aseparating column and an electron-capture detector.

A downstream shut-off valve 30 is provided to isolate the detector andcolumn from the external atmosphere, to which it is normally coupledthrough a detector vent 32. r

The probe 16 normally applies samples of gas through the sample valveand a sample loop to a pump unit 34 (not shown) which is coupled througha pump vent 36 to atmosphere. A pump switch 38 controls the energizationof the pump 34. A flow meter control valve 40 selectively applies thesampled gas stream either through a flow meter 42 or through the pump 34and pump vent 36, to atmosphere. Additional gas subsystem controls areaccessible from the back panel, and are identified in connection withthe description of FIG. 2 below. A probe flush valve 44 is provided topermit the carrier gas supply to be admitted to the probe assembly 16for leak rate detection.

In order to permit the continuous sampling mode of operation, acontinuous sampling enabling valve 46 is interposed between the pump 34output and the input to the detector at the output of thechromatographic column. A bypass valve 48 connects the atmospherethrough a restrictor, to the intake side of the pump. This alternativeintake source is available to prevent overload on the pump in the eventthat the volumetric flow through the probe is insufficient during modesin which the predominating probe intake is the carrier gas being flushedinto the probe and the continuous sampling enable valve 46 is closed.

Various elements of the electrical subsystem are also found on the frontpanel 12. A mode switch 50 alternatively selects a normal mode, a firstintegrating mode or a second integrating mode. The mode switch 50 is athree-position switch for that purpose. An amplifier zeroing control 52and a detector zeroing control 54 are also provided to permit the nullbalancing required for both normal and integrating modes. A supplementaldetector range switch 56 enables a greater range of control for thedetector for operation in a continuous sampling mode when the effects ofthe oxygen in the atmosphere must be compensated.

A telltale lamp 58 signals when the batteries of the self-containedpower supply are being recharged. A battery-select switch 60 permits oneof two storage batteries that are provided to be connected as the sourceof power. A reset switch 62 provides a short-circuit feedback loop inthe amplifier for nulling purposes. A meter 64 is provided foradjustment. calibration, 'and test purposes as well as display of theoutput signal. An appropriate meter zero control 66 is also provided tonull a driving amplifier for the meter 64 prior to operation.

A range multiplier switch 68 permits the selection of one of a number ofpossible sensitivities and also certain limited operating configurationscan be chosen. A meter zero position is provided to enable a zeroing ofthe meter 64 with the meter control 66. A second position permits acheck of the selected battery to determine whether sufficient powerremains in the battery for operation before recharging is required.Three other positions select sensitivity ranges, based upon the expectedconcentration of the tracer gas. An 0 inhibit switch 70 is provided toselectively include an amplifier disabling circuit that is energizedwhen the sample switch 28 is closed so that the detector and amplifiercircuits will not respond to the detection of oxygen gas, which also hasa substantial electron capture capability in its normal concentrations.By placing the O inhibit switch 70 in the off" position, this internalcircuitry is disabled, and the instrument will respond to oxygen peaksin the chromatogram, as well as the peaks attributable to selectedtracer gases.

Appropriate outputs are provided including a headsetjack 72 and arecorderjack 74. A volume control 76 adjusts the volume of the signalwhich is applied to the headset. A power, on-off switch 78 enables theoperation of the electrical circuit.

Turning next to FIG. 2, the back of the instrument, includes arecharging unit 80 with an appropriate power cord 82 that is adapted tobe connected to a source of 60-cycle 1 lO-volt AC voltage. A smallbottle 84 of carrier gas is installed in an appropriate mountingbracket. The carrier gas control knob 22 and the carrier gas pressuregauge 24 on the front panel are appropriately interconnected to the gasbottle 84. A pressure regulator (not shown) limits the rate of flow ofthe carrier gas. A gas turn on valve 86 provides the basic control ofthe gas supply. A continuous flow adjust valve 88 regulates the relativeamounts of gas going to the vent 36 and the detector 98 through thecontinuous sample enabling valve 46.

Turning next to FIG. 3, there is shown in substantially block form, thegas-handling subsystem of the detector and analyzer of the presentinvention. As can be seen, the carrier gas container 84 is connectedthrough a regulator 80 and the available pressure is monitored by thepressure gauge 24.

The carrier gas normally proceeds through a trap 92 which removescontaminants, and then into a sample valve 94. The sample valve 94(shown in greater detail in FIG. 7 below) normally connects the carriergas supply to a chromatographic column 96 and into an electron capturedetector 98. The downstream shut-off 30 applies the gas flow from thedetector 98 to the detector vent 32. For one mode of leak-ratedetection, an alternate carrier gas path is provided through the probeflush valve 44 and the adjustable, probe flow restriction 26 into theprobe 16.

The sampled gas path includes an intake conduit 100 from the probe 16which is connected to the sample valve 94. Normally, the sample valve 94carries the intake from the probe 16 to a sampling loop 102 which isconnected to a sampled gas outlet line 104.

The flow meter enable valve 40 alternatively connects the sampled gasoutlet line 104 to a first path 106 which includes a pump 34 and thepump vcnt 36, or to a second, parallel path 108, which includes the flowmeter 42. The parallel paths 104, 106 are both connected to the vent 36.The first path 106 also includes an alternative input path fromatmosphere through intake bypass valve 48 and a restrictor 110, which isopen to atmosphere.

In the normal, gas detection mode, the Probe Flush valve 42 is closedand the Flow Meter Enable valve 40 couples the first parallel path 106to the sampled gas outlet path 104. The bypass valve 48 may be opened toprovide a supplementary flow of air to the pump 34, if it is deemedundesirable to draw all of the pump intake through the probe 16.

In alternative configurations, the Flow Meter Enable valve 40 directsthe flow of sampled gas through the Flow Meter 42 in the second parallelpath 108. During leak rate measurements or system calibrationmeasurements, the rate of gas flow is important. When the carrier gas isapplied to the probe 16 through the Probe Flush valve 44, the rate offlow can be monitored on the flow meter 42 and the flow can be modifiedby the flow adjustment valve 26. This configuration will also be used inthe special case of calibrating the instrument through the use of acommercially available calibrated leak source." In these modes, thebypass valve 48 is normally closed.

When the sample push button 28 is operated, the sample valve 94configuration changes to include the sample loop 102 into the maincarrier gas stream and to provide a bypass for the sampled gas flow fromthe probe 16 to the sampled gas outlet 104. The chromatographic column96 operates to separate the various component elements mixed in thecarrier gas stream. The elution'time of each component differs dependingupon the chemical nature of the component. The component gases thusseparated are then successively applied to the electron capture detector98.

In the continuous sampling mode, the probe flush valve 44 is closed, thebypass valve 48 is closed, the continuous sampling enabling valve 46 isopened and the pump 34 is energized. A large volume of air is drawnthrough the probe 16 through the normal, sampled gas flow path. Carriergas is continuously applied through the sample valve 94 and thechromatographic column 96 to the detector 98. Part of the pump 34 outputis fed back, through an alternative flow path, through the continuoussample valve 46 directly into the carrier gas stream between the column96 and the detector 98. The volume of sampled air applied to thedetector 98 is determined primarily by the setting of the continuoussample adjust valve 88, which passes substantially all the pump 34output to atmosphere through pump vent 36. By decreasing the volume ofsampled air through the continuous sample adjust valve 88, the volumethrough the detector 98 can be increased.

Turning next to FIG. 4, there is shown in partly block, partly schematicdiagram, the electronic subsystem 112 of the instrument of the presentinvention. An electron capture detector 98, such as is shown in theabove-mentioned patent to Loveiock, US. Pat. No. 3,247,375, or asillustrated in FlG. 12 of the Lovelock publication at page 171 of Vol.33 of Analytical Chemistry, for February, 1961, may be used.

In the preferred embodiment, however, a cylindrical detector isprovided, the inside surface of which is lined with a tritium foil,which is a radioisotope and a beta emitter. The inside surface can beconsidered the cathode of the detector and an appropriate connection ismade to an electrometer amplifier 114 or power supply 116. A concentric,coaxial, needle-like probe, electrically isolated from the cathode isplaced inside the cylinder and is connected as the anode of thedetector. The flow of gas through the detector is in the axialdirection. Obviously, the connections can be reversed in appropriatecircumstances, with probe functioning as the cathode.

The detector anode is connected to the power supply circuit 114 whichincludes a crystal-controlled oscillator 118 and a blocking oscillator120.

In has been found that the electron capture detector envisaged byLocklock, supra, is frequently sensitive. Accordingly, errors can beintroduced by frequency drifts or by other instabilities of the pulsegenerating circuitry. Accordingly, a highly stable pulse source isprovided in the present invention. In the preferred embodiment, acrystalcontrolled oscillator 118 is utilized, which can be held to frequencystabilities greater than one part in 10'. It is believed that such anextremely stable pulse source is essential for the reliable operation ofthe gas detector of the present invention.

In the preferred embodiment, the power supply circuit 116 is adjusted toprovide relatively positive pulses approximately 30 volts in magnitudeand approximately 3 microseconds in duration at microsecond intervals.In one gas detector that was built according to the present invention, a100 microsecond pulse interval was used with satisfactory results.

The cathode surface radiates a steady, uniform supply of electrons andalso produce additional electrons in the carrier gas all of which flowthrough the carrier gas environment in the detection chamber to theanode, so long as a potential gradient exists as between theseelectrodes. in the pulsed mode of operation, the electrons reach theanode only during the existence of the pulse, at which time thepotential gradient exists.

By appropriate filtering circuits, a steady-state, quiescent current isprovided from the detector 98 to the electrometer amplifier 114. 1n thenormal condition, this current is balanced by a baseline adjustmentcircuit 122 which includes the detector zeroing potentiometer 54 adaptedto connect between a source of potential 124 of opposite polarity tothat of the pulsed power supply 116 and a source of common referencepotential 126. A dropping resistor 128 of approximately the sameresistance as the maximum potentiometer 54 resistance is connectedbetween the sources 124, 126. A large resistor 130 in series with thepotentiometer tap then applies a baseline compensating current to thedetector 98 so that the input to the electrometer amplifier 1 14 iseffectively zeroed.

It is then apparent that whenever pure carrier gas is flowing throughthe detector 98, any electrical current that flows will be equalled andbalanced by the setting of the Baseline Adjustment circuit 122 and thata zero input will be applied to the electrometer amplifier 114.Similarly, in the continuous sampling mode, the baseline current can beincreased to balance the additional current from the normal atmospherecomponent that is added to the carrier gas stream.

Whenever a tracer gas, such as SP or other electron capturing compoundis present in the carrier gas stream, some of the free electrons will becaptured and the average current produced in the detector 98 will becorrespondingly reduced. It is this incremental current (which isproportional to the concentration of the electron absorbing compound)that is amplified in the electrometer amplifier 114 and, as amplified,provides the signal output representing the presence of additionalelectron capturing compounds in the carrier gas stream.

The mode select switch 50 of the front panel selectively includes one ofthree alternate feedback paths between the output and the input of theelectrometer amplifier 114. A first, or normal feedback path 132includes a resistor 134, capacitor 136 in parallel combination toprovide a predetermined transfer impedance and time constant. Themagnitude of the feedback resistor 134 determines the amplification ofthe signal.

A second or Integrate 1 feedback path 138 includes a first integratingcapacitor 140. A third path or Integrate 2 feedback path 142 includes asecond integrating capacitor 144. The integrating capacitors permitcharge storage so that the current in the tracer peak may be integratedand stored for a brief period of time. This optional mode provides aquantitative measurement unaffected by the form factor of the peak(i.e., it is the total absorbed charge that is measured, not the rate ofcharge absorption, as measured in the normal linear mode).

In a typical instrument embodying the present invention, the firstintegrating capacitor 140 had a capacitance value that differed from thesecond integrating capacitor 144 by a factor of 10, thereby furnishingtwo sensitivity ranges that differed by an order of magnitude.

A fourth or short circuit" feedback path 146 is also provided. Thefourth path 146 has a relay control switch 148 interposed therein. Thefourth path 146 is used for adjusting the amplifier and/or the baselineadjustment circuit and is also utilized in the oxygen blanking" circuit.The switch 148 is controlled by a relay 149 that is alternativelyenergized by the Reset button 62 or by the Sampling switch 28.

The Reset button 62 connects the relay 149 drive coil directly to asource of potential 150. The Sampling switch 28 triggers an OxygenBlanking Circuit 152 which connects the relay 148 drive coil to thesource of potential 150. The Blanking circuit 152 includes a more orless, conventional, one-shot-type circuit that responds to an inputimpulse to provide an output impulse of predetermined duration andmagnitude.

The Oxygen Blanking Circuit 152, in order to operate, must be enabled bythe O inhibit switch 70 being in the ON position. It will be noted, thatby energizing the relay 148 and closing the short circuit loop 146, anycharge that may have been stored in the integrating capacitors 144, 140,if in one of the Integrate modes, will be dumped. The reset switch 62can therefore be used to zero the system output.

An output divider 154 is connected to the Range switch 68 to providealternative outputs of varying potential to a meter circuit 156. Theproper range setting will provide a suitably limited output to the metercircuit 156. The meter circuit 156 includes an amplifier 158 and themeter 64 which is displayed on the front panel 12.

Alternative outputs are provided directly on a first output line 162which is adapted to be connected to an external graphic recorder throughthe output terminal 74, and on a second output line 164 which isconnected to a voltage-controlled oscillator 166. The voltage controlledoscillator 166 is adapted to provide an audiofrequency signal to aheadset 168 through the output jack 72 at a first frequencycorresponding to a null or zero output. Any increase in the outputfrequency is then a function of the magnitude of the electrometeramplifier 114 output signal, and permits operation of the device on anaudio basis.

Turning next to FIG. 5, there is shown a sectional view of an improvedprobe 16 adapted for use in the present invention. The improved probe 16includes an elongated cylindrical shell 170 that is flared at an openend to a conical shape 172. The pliable resilient collar fits snugly onthe conical portion 172 and is intended to provide a substantially gastight seal when held against a surface whose integrity is suspect.

Within the cylindrical portion of the probe 16 are a pair of tubes. Afirst tube 174 is coupled to the carrier gas supply and provides flow ofcarrier gas to operate in the Leak Rate Detection mode. A second tube176 is connected to the sampling valve 92 and is the intake to thesampling system. The flexible tube 18 includes a first passage 178 whichcommunicates with the first tube 174, a second passage 180 thatcommunicates with the second tube 176.

In FIG. 6, including FIGS. 6a and 6b, there are shown in diagrammaticform, the flow paths provided by the sampling valve. In FIG. 6a, thevalve is in its normal, non-sampling configuration and the carrier gaspath goes through the valve 94 without interruption. The sample gas pathproceeds from the intake conduit 100 through the sampling loop 102 tothe sampled gas outlet line 104.

When the sampling valve 94 is actuated to acquire a sample for detectionand analysis, the carrier gas path is re-routed, as shown, in FIG. 6bthrough the sampling loop 102. The sampled gas path then leads directlyfrom the input conduit 100 to the sampled gas output line 104, throughthe valve 94. Yet other valves can be devised which will accomplishedthe desired results, and FIG. 6 is to be deemed illustrative, only.

FIG. 7 is a block diagram of the portable power supply portion of thepreferred embodiment of the present invention, and includes a normalelectrical plug 182 adapted to be connected to a source of AC power. Thebattery recharger is connected to the plug 182 and, through appropriateswitches, is alternatively connected to a first or second battery 184,186. The battery select switch 60, connects one of the batteries as asource of power to a static inverter 188 which serves as the basic, DCpower supply for the circuits of FIG. 4. The supply is operable in fourmodes as indicated in the table below.

Turning next to FIG. 8 there are shown typical calibration charts foruse with the apparatus of the present invention. In FIG. 8(a) utilizingthe calibration curve 190, leak rate can be measured directly from theresponse of the output meter 64 when set, for example, at apredetermined range (range 10). A signal of 0.6 mA would correspond toan SF concentration of 0.1 ppb and a leak rate of I X 10*" Atm. cc/scc.

With FIG. 8(b), using the calibration curve 192, the identicalconcentration and leak rate would correspond to a reading of 0.18 mA inthe Integrate 1 mode, or a reading of 118 mA in the Integrate 2 mode butin the next higher range (range It is clear that still other conversionsare possible and'that other tables, charts and graphs can be devised.

Finally, in FIG. 9, there is shown a typical chromatogram 200 that maybe produced by the instrument, operating in the normal mode with the Oblanking circuit disconnected. If the sample valve is energized at timeT1, thereby including the sample loop in the carrier gas stream,approximately ten seconds later, the first peak 210 representing oxygen,will be noted, which generally will drive the meter off scale because ofthe relatively high concentration of oxygen in any atmosphere sample. Asecondary oxygen peak 212 may be produced by a minute valving dischargeafter the initial sample insertion. The undersliot 213 is a commonsaturation overload effect for electron capture detectors generally.Approximately 60 seconds later, a pulse 213 representing SF, may beencountered.

With a second sampling, at T the oxygen peaks 210' and 212 appear,substantially of the same amplitude. However, a higher SP peak 214' isnoted,'indicating a relatively higher concentration of SF In thesampling interval initiated at T the SF peak 214" is of substantialmagnitude, signifying a substantial concentration of sF in the vicinityofthe probe.

We claim:

1. in a portable, self contained gas detector for signalling thepresence of electron capturing compounds in excess of predeterminedthreshold amount including a portable, self contained source ofelectrical power,

a portable supply of carrier gas, and

means for introducing limited amounts of a gaseous atmosphere under testinto a present flow of carrier gas, circuits for detecting andsignalling the presence of electron capturing compounds, comprising incombination:

a. a precision, crystal controlled oscillator power supply, coupled tothe electrical power source for applying precise increments ofelectrical energy at a predetermined rate;

b. an electron capture detector having first and second electrodes, oneof said electrodes including a radioactive source for ionizing gasespassing therethrough;

0. means coupling one of said detector electrodes to said power supply,for periodically creating a potential gradient as between theelectrodes, at the predetermined rate;

baseline current supply means coupled to the other of said detectorelectrodes, adjustable to provide a biasing current sufficient to equalthe current as between the electrodes in the presence of an appliedreference carrier gas mixture; and

e. electrometer amplifier means, connected to said other of saiddetector electrodes for amplifying any electrical output therefrom,

whereby any change in the electron capturing properties of the gasmixture applied to said detector produces an amplified output signal atsaid amplifier means.

2. Apparatus of claim 1, above, wherein said electrometer amplifiermeans includes a first feedback loop having a parallel combination ofresistance and capacitance for providing gain to said amplifier means.

3. Apparatus of claim I, above, wherein said electrometer amplifiermeans include a short circuit feedback loop, for adjusting saidamplifier means in the absence ofa signal input from said electroncapture detector.

4. Apparatus of claim 1, above, wherein said electrometer amplifiermeans include an integrating feedback loop for providin a continuousoutput signal representing an average applied electron capture detectoroutput signal.

5. Apparatus of claim 1, above, further including means coupling saidelectrometer amplifier means to a meter for visible display.

6. Apparatus of claim 1, above, further including means coupling saidelectrometer amplifier means to a recorder to provide a graphic record.

7. Apparatus of claim 1, above, further including means coupling saidelectrometer amplifier means to a voltage controlled audio oscillator toprovide an audible signal corresponding to said detector output.

8. Apparatus as in claim 1, above, wherein said power supply is crystalcontrolled to stahilitics greater than one part in 10.

9. Apparatus as in claim 1, above, wherein said power supply pulses areof approximately 3 usec duration at I00 usec intervals.

10. A portable, self-contained gas detector for signalling the presenceof electron capturing compounds in excess of predetermined thresholdamounts comprising in combination:

a. a portable, self-contained source of electrical power;

b. a portable supply of carrier gas;

c. mixing means coupled to a source of a gaseous atmosphere under testand said supply of carrier gas for introducing limited amounts of thegaseous atmosphere under test into a present flow of carrier gas;

. separating means connected to said mixing means, including achromatographic column for selectively delaying the passage therethroughof the constituents of the output of said mixing means;

e. an electron capture detector adapted to be connected to said sourceof electrical power and said test atmosphere gases and for signalling asto each constituent the electron capturing capability, relative to apredetermined threshold based on a selected gaseous mixture; and

f. electrometer amplifier means coupled to receive the output of saidelectron capture detector for amplifying the output signals thereof,said electrometer amplifier means including blanking circuit means,coupled to said electrometer amplifier means, for disabling saidamplifier means for a selected time interval,

whereby electron capture detector output signals corresponding to thepresence of atmospheric oxygen in said electron capture detector aresuppressed to permit subsequent generation of lesser magnitude signalsrepresenting the presence of other electron capturing compounds inlimited quantities.

1. In a portable, self contained gas detector for signalling thepresence of electron capturing compounds in excess of predeterminedthreshold amount including a portable, self contained source ofelectrical power, a portable supply of carrier gas, and means forintroducing limited amounts of a gaseous atmosphere under test into apresent flow of carrier gas, circuits for detecting and signalling thepresence of electron capturing compounds, comprising in combination: a.a precision, crystal controlled oscillator power supply, coupled to theelectrical power source for applying precise increments of electricalenergy at a predetermined rate; b. an electron capture detector havingfirst and second electrodes, one of said electrodes including aradioactive source for ionizing gases passing therethrough; c. meanscoupling one of said detector electrodes to said power supply, forperiodically creating a potential gradient as between the electrodes, atthe predetermined rate; d. baseline current supply means coupled to theother of said detector electrodes, adjustable to provide a biasingcurrent sufficient to equal the current as between the electrodes in thepresence of an applied reference carrier gas mixture; and e.electrometer amplifier means, connected to said other of said detectorelectrodes for amplifying any electrical output therefrom, whereby anychange in the electron capturing properties of the gas mixture appliedto said detector produces an amplified output signal at said amplifiermeans.
 1. In a portable, self contained gas detector for signalling thepresence of electron capturing compounds in excess of predeterminedthreshold amount including a portable, self contained source ofelectrical power, a portable supply of carrier gas, and means forintroducing limited amounts of a gaseous atmosphere under test into apresent flow of carrier gas, circuits for detecting and signalling thepresence of electron capturing compounds, comprising in combination: a.a precision, crystal controlled oscillator power supply, coupled to theelectrical power source for applying precise increments of electricalenergy at a predetermined rate; b. an electron capture detector havingfirst and second electrodes, one of said electrodes including aradioactive source for ionizing gases passing therethrough; c. meanscoupling one of said detector electrodes to said power supply, forperiodically creating a potential gradient as between the electrodes, atthe predetermined rate; d. baseline current supply means coupled to theother of said detector electrodes, adjustable to provide a biasingcurrent sufficient to equal the current as between the electrodes in thepresence of an applied reference carrier gas mixture; and e.electrometer amplifier means, connected to said other of said detectorelectrodes for amplifying any electrical output therefrom, whereby anychange in the electron capturing properties of the gas mixture appliedto said detector produces an amplified output signal at said amplifiermeans.
 2. Apparatus of claim 1, above, wherein said electrometeramplifier means includes a first feedback loop having a parallelcombination of resistance and capacitance for providing gain to saidamplifier means.
 3. Apparatus of claim 1, above, wherein saidelectrometer amplifier means include a short circuit feedback loop, foradjusting said amplifier means in the absence of a signal input fromsaid electron capture detector.
 4. Apparatus of claim 1, above, whereinsaid electrometer amplifier means include an integrating feedback loopfor providing a continuous output signal representing an averaged,applied electron capture detector output signal.
 5. Apparatus of claim1, above, further including means coupling said electrometer amplifiermeans to a meter for visible display.
 6. Apparatus of claim 1, above,further including means coupling said electrometer amplifier means to arecorder to provide a graphic record.
 7. Apparatus of claim 1, above,further including means coupling said electrometer amplifier means to avoltage controlled audio oscillator to provide an audible signalcorresponding to said detector output.
 8. Apparatus as in claim 1,above, wherein said power supply is crystal controlled to stabilitiesgreater than one part in
 107. 9. Apparatus as in claim 1, above, whereinsaid power supply pulses are of approximately 3 Mu sec duration at 100Mu sec intervals.